It has been known for some time that rare earth doped glasses in fiber form could be used as a lasing medium. However, it has only been recently that the possibility of using such fibers as the amplification medium in an optical fiber communication system has begun to be explored seriously. Most interest is currently directed towards fiber that comprises erbium ions. Among possible pump wavelengths (.lambda..sub.p) are those in the 0.8 to 1.0 .mu.m range (e.g., 0.98 .mu.m) and those relatively close to (but below) the anticipated signal radiation wavelength .lambda..sub.s of about 1.5 .mu.m (e.g., 1.48 .mu.m). See, for instance, P. Urquhart, I.E.E. Proceedings, Vol. 135(part J, No. 6), pp. 385-407, Dec. 1988.
The principles of amplification of an optical signal in an Er-doped fiber amplifier are known to those skilled in the art. See, for instance, J. R. Armitage, Applied Optics, Vol. 27(23), pp. 4831-4836, Dec. 1988. Efforts have been undertaken to optimize the characteristics of Er-doped fiber amplifiers. See, for instance, U.S. Pat. No. 4,923,279, which discloses a single mode optical fiber having a core whose inner region contains the dopant and an outer region which is substantially dopant free. The matching of the dopant distribution and the signal mode field can reduce the pump threshold for a laser and improve the gain performance for a given pump power.
Although optical fiber amplifiers as described above can advantageously be used in a variety of communication systems, there are many potential applications for an optical amplifier to which fiber amplifiers are not readily or conveniently adapted. For instance, it would be desirable to be able to integrate an optical amplifier with electronic devices or circuits, since such integration can be expected to result in decreased cost, increased ruggedness, and possibly greater speed. Such integration would be facilitated by the availability of a planar optical amplifier. Furthermore, the availability of planar optical amplifiers would significantly advance progress towards fully integrated optics and, on a somewhat less advanced level, have immediate applications in such diverse fields as optical signal detection, optical backplanes of switching equipment, local area optical fiber networks, and optical fiber cable TV systems. Finally, planar optical amplifiers can be expected to be desirable replacements for optical fiber amplifiers, due to their more compact nature and increased ruggedness, and are likely to find application in undersea lightwave communication systems.
Planar optical waveguides are known. Among the known types of planar waveguides are silica-based glass waveguides disposed on a silicon substrate, as disclosed, for instance, in U.S. Pat. No. 4,902,086, incorporated herein by reference. For a broader discussion of planar optical waveguides, see, for instance, "Integrated Optics, Physics and Applications", edited by S. Martellucci et al., Plenum Press, especially the chapter by G. Chartier, pp. 49-72. On page 53 of that monograph can be found a compilation of materials in which optical waveguides have been formed, together with a listing of fabrication techniques used. On pages 63 to 65 of the monograph can be found a discussion of waveguide formation by means of ion implantation. As described there, the technique involves exposing a substrate to a collimated ion beam, resulting in modification of the properties of an appropriately shaped region of the substrate that manifests itself as a change in refractive index of the region.
P. J. Chandler et al., Electronics Letters, Vol. 26(5), pp. 332-334 (March 1990) report on the fabrication of optical waveguides by implantation of inert ions (He.sup.+) into single crystal LiNbO.sub.3 bodies. The LiNbO.sub.3 was uniformly doped with MgO, or Nd and Cr.
P. J. Chandler, et al., Electronics Letters, Vol. 25(15), pp. 985-986 (July 1989) report formation of a planar waveguide laser by implantation of He.sup.+ ions into a standard laser crystal (Nd:YAG).
M. Yamaga et al., Japanese Journal of Applied Physics, Vol. 25(2), pp. 194-199 (February 1986) prepared Nd-doped garnet films on single crystal YAG substrates by sputter deposition.
Y. Hibino, I.E.E.E. Photonics Technology Letters, Vol. 1(11), pp. 349-350 (November 1989) disclose formation of a Nd-doped silica optical waveguide laser on a Si substrate by a technique that involves depositing glass soot onto the substrate, soaking the soot layer in Nd-containing alcohol, sintering of the soot layer, patterning of the sintered layer to form a 20 .mu.m wide core ridge, and overcladding the thus formed core strip with a glass cladding that was also formed from soot. Fluorescence and lasing were observed.
R. Brinkman et al., Proceedings of the Integrated Photonics Research Conference, Hiltonhead, S.C., Mar. 26-28, 1990, pp. PD-1 to PD-2, report manufacture of annealed Er-implanted single mode waveguides in single crystal LiNbO.sub.3. The manufacturing process comprised implanting 10.sup.16 cm.sup.-2 Er ions at 200 keV into the LiNbO.sub.3 surface. The resulting very shallow Er distribution (50 nm width, peak at 70 nm depth) was then spread out by means of an annealing treatment. The anneal also resulted in epitaxial re-growth of the amorphized surface layer of the LiNbO.sub.3 substrate. After a 45 hour anneal the maximum of the Er distribution (6.times.10.sup.19 cm.sup.-3) occurred at the substrate surface, with a 1/e penetration depth of 1.8 .mu.m. In thus prepared Er-doped LiNbO.sub.3 samples waveguides were formed by Ti indiffusion, as well as by proton exchange. The resulting waveguides apparently were about 95 nm deep. Thus, the Er distribution was essentially constant throughout the waveguide. Fluorescence was observed in the thus produced waveguides.
In view of the potentially high commerical significance of planar optical gain devices (e.g., amplifiers and lasers), improved such devices would clearly be of interest. In particular, devices having improved matching of the dopant atom distribution to the mode field of the signal radiation in the planar waveguide of the device would be very desirable, since such improved matching can, inter alia, result in improved gain performance of the device. This application discloses a waveguide with such improved matching, and a technique for making the waveguide.